Alkoxyalkanoic acids are useful as anionic surfactants or emulsifying agents. These acids, being composed of only the elements C, H and O, do not pose the environmental problems that other detergents containing heteroatoms such as N, S, and P pose. The alkoxyalkanoic acids can be prepared in a two-step process of first reacting an alkanol with an alkoxylate and a suitable alkoxylation catalyst and thereafter converting the resultant alkoxyalkanol to the alkoxyalkanoic acid.
It is also known to convert alkoxyalkanols such as methyl carbitol to the corresponding carboxylic acids by oxidizing them with nitric acid. However, relatively large amounts of nitric acid are required and not all of the nitric acid can be separated by distillation. In addition, cleavage of the ether linkages occurs to a large degree during this process.
Japanese Patent No. 50-96516, issued Jul. 31, 1975, discloses a process for the preparation of carboxylic acid salts by the liquid phase dehydrogenation of alcohols with caustic alkali in the presence of precious metal catalysts, including palladium. This process uses a relatively high temperature, 100.degree. C.-270.degree. C. These high temperatures can degrade the ether linkages especially in the highly ethoxylated alcohols.
It is known to use nitroxyl radicals/oxoammonium salts in the oxidation of primary alcohols to produce aldehydes and acids and secondary alcohols to ketones. Journal of Organic Chemistry, vol. 52 (12), pp. 2,559-2,562; Pure and Applied Chemistry, vol. 62(2), 1990, pp. 217-222; Journal of Organic Chemistry, vol. 55, 1990, pp. 462-466. The primary products produced in these processes are aldehydes and the stoichiometrically consumed oxidant is hypochlorite.
It is generally more difficult to oxidize alkoxyalkanols than alkanols as it is difficult to oxidize alkoxyalkanols without splitting the molecular chain at the ether linkage and thereby produce a large proportion of undesired by-product. In addition, the more oxyalkylene groups there are in an alkoxyalkanol the more difficult it is to selectively oxidize. Thus, lower concentrations of the desired hydroxyl groups are present as the molecular weight increases. This consequently increases the probability of cleavage at the ether linkages.
It would therefore be advantageous to produce alkoxyalkanoic acids in high yields and with high selectivities from alkoxyalkanols having a large number of oxyalkylene groups without producing large amounts of other products such as aldehydes, esters, polyethylene glycols and fatty acids.
It has been found that alkoxyalkanoic acids can be produced in high yields and with high selectivities without forming highly corrosive, difficult to separate, side-products by using catalytic amounts of a stable free radical nitroxide and a NO.sub.x -generating compound.